Controls on diel metal cycles in a biologically productive carbonate-dominated river

Abstract

The 24-hourcycle in solar radiation and subsequent primary productivity by submerged vegetation controls the diel cycles of trace metal concentrations in streams directly, via photochemical reactions and autotrophic assimilation, and indirectly through changes in mineral saturation states, redox conditions, and adsorption reactions. Most prior studies have focused on streams with elevated metal concentrations and thus cycling is not as well understood in pristine streams where the effects of submerged plant metabolism may be critical in controlling the availability and diel cycling of metals. To assess controls on diel metal cycling in an alkaline river with naturally low metal concentrations, water samples were collected every 1–2h for>24hours in three seasons between March 2009 and May 2010 in the Ichetucknee River, north-central Florida (USA), a large (Q>6m3/s), spring-fed river flowing over a carbonate karst terrain. Ca2+, Ba, Fe, and U concentrations exhibit statistically significant diel cycles regardless of season, while Mn, and possibly Sr, cycles were present only in the spring. Mn, Ba and Ca2+ concentrations increased at night, out of phase with the cycles in pH and carbonate saturation state. Daytime precipitation of calcium carbonate could partially control Mn, and to a lesser extent Ba, cycles through co-precipitation. Fe and U concentrations cycled in phase with solar radiation, pH, and DO concentrations, reaching maxima in late afternoon. Adsorption of cationic Mn and Ba species and anionic U and Fe species could explain the opposing cycles of these two sets of metals as could the photo-reduction of Fe, enhanced by the high water clarity. The diel cycles of Mn, Fe and Ba also appear to be directly controlled by autotrophic assimilation based on estimates of net carbon fixation by submerged vegetation and the metal stoichiometry of the dominant submerged macrophyte and algae in the river. These results demonstrate how multiple biogeochemical processes resulting from solar radiation and associated aquatic plant metabolism likely control diel and seasonal cycles of metals and illustrate the importance of these linked processes in mediating the water quality and environmental availability of metals in pristine streams with naturally low metal concentrations.